FIG. 1 is an electric diagram illustrating an overvoltage protection circuit 1 disclosed in United States Patent Application Publication No. 2015/02221628. Circuit 1 is connected between two input/output terminals, IO1 and IO2, of a circuit to be protected and a reference terminal GND. Circuit 1 comprises, in series, between input/output terminal IO1 and reference terminal GND, a forward diode 3 and a diode 5 with a reverse-controlled avalanche voltage. The protection circuit further comprises, in series, between input/output terminal IO2 and reference terminal GND, a forward diode 7 and a diode 9 with a reverse-controlled avalanche voltage. The signal on terminals IO1 and IO2 may for example be a binary data signal. In the case of an overvoltage on terminal IO1, said overvoltage is removed by diode 3 and diode 5. The overvoltage is then limited to a voltage greater than the normal positive voltages of the data signal. In the case of an overvoltage on terminal IO2, said overvoltage is removed by diode 7 and diode 9. The overvoltage is then limited to a voltage greater than the normal positive voltages of the data signal.
It is desirable for the protection circuit to influence neither the operation nor the performance of the circuit to be protected, nor the shape of the supplied signals. Particularly, if the signal on the terminal to be protected is a binary square-pulse signal, to keep the square shape of this signal, the total capacitance of the protection circuit should be as low as possible.